The present invention pertains to audio processing systems and, in particular, to an audio processing system for use in an amplitude modulated transceiver. A number of audio amplifier systems for use in radio frequency tranceivers have been developed in the prior art. Such audio systems normally serve two functions. Firstly, the audio system processes the audio output from the tuner portion of the receiver such that the signal is of sufficient magnitude for driving a speaker. Secondly, the audio system is often incorporated in the transmitter, whereby an information signal, such as the signal produced from a microphone input, is amplified through the audio system and used to amplitude modulate a radio frequency carrier signal. Since the audio system is used in both the receive and transmit modes, schemes have been developed for the proper switching of the audio system between the receiver and transmitter.
A problem recognized by prior art audio systems is that the dynamic range of audio signals, such as those originating from a microphone, may produce audio peaks which, when used in amplitude modulated transmitters, result in overmodulation of the transmitter. Overmodulating the transmitter is undesirable since it results in a distorted audio signal as well as the possibility of adjacent channel splatter. This problem may be remedied by reducing the audio modulating signal to the transmitter, but this results in a lower average modulation level of the transmitted signal and thus a smaller effective radiating area of the transmitter.
To remedy the above described problem some prior art transceivers have incorported audio compressors which compress the audio modulating signal before applying it to the transmitter. As its name implies, the audio compressor acts to reduce the dynamic range of audio source material. Thus, the transmitted signal may be modulated at a higher average level, thereby improving the transmitter's effective radiating area, while maintaining the modulation level below a set maximum limit.
A significant problem heretofore unresolved in the prior art occurs in the transceiver's receive mode. The receiver often processes signals of very weak, then very strong, amplitude. This results in a wide average level of audio signal reproduced by the speaker, which becomes an annoyance to a listener. For example, when tuned to one transmitting station the received signal might be very small and thus the operator would have to increase the volume on his receiver. However, a subsequently received broadcasting station might be received at a very high radio frequency level resulting in not only an extremely loud speaker output, but also possible clipping, and thereby distortion, of the audio stage. While many receivers employ automatic gain control circuitry which tends to minimize this effect, it is still quite a nuisance in conventional radio receivers, since rated audio is commonly measured at 30% modulation while signals may be processed which are near 100% modulation, i.e. a 10db level above the 30% level. Even with an ideal AGC system this can result in overdriving one or more stages of the audio chain thereby resulting in both increased volume and distortion.